Semiconductor Device

ABSTRACT

A semiconductor device and a method of making the same. The device includes a semiconductor substrate having an AlGaN layer on a GaN layer. The device also includes first contact and a second contact. The average thickness of the AlGaN layer varies between the first contact and the second contact, for modulating the density of an electron gas in the GaN layer between the first contact and the second contact.

BACKGROUND OF THE INVENTION Semiconductor Device

This invention relates to a semiconductor device. In particular, thisinvention relates to a semiconductor device having a semiconductorsubstrate including an AlGaN layer on a GaN layer.

In recent years, GaN High Electron Mobility Transistor (HEMT) deviceshave drawn a lot of attention regarding their high potential to replaceSi or SiC for use as High Voltage (HV) devices. GaN HEMTs are typicallyfabricated by applying ohmic source and drain contacts and a Schottkygate contact on top of an epitaxially grown structure including an AlGaNbarrier layer on a GaN channel layer.

As AlGaN is a piezoelectric material, the lattice mismatch between GaNlayer and the AlGaN layer gives rise to a potential difference over thebarrier, which modulates the band structure in such a way that a quantumwell filled by two dimensional electron gas spontaneously forms in theGaN near the AlGaN/GaN interface. The high mobility of this electron gasleads to devices having very low resistance compared to other kinds ofField Effect Transistor (FET). As with other FETs, the conductivity inthe channel can be modulated by the application of a potential to thegate.

GaN HEMT devices find application as RF power devices in areas wherehigh gain and low noise are required at high frequency. They offer theadvantages of higher efficiencies, larger bandwidth and largertemperature range over Si LDMOS and of higher polarization voltage overGaAs HEMT devices. Moreover, GaN HEMT devices start to penetrate e.g.the power conversion market in the voltage range from 50V to 600V, wherethe GaN devices offer very low specific on-resistances compared to Sibased contenders and at lower prices than the competing wide-band-gapmaterial SiC devices.

SUMMARY OF THE INVENTION

Aspects of the invention are set out in the accompanying independent anddependent claims. Combinations of features from the dependent claims maybe combined with features of the independent claims as appropriate andnot merely as explicitly set out in the claims.

According to an aspect of the invention, there is provided asemiconductor device including a semiconductor substrate having an AlGaNlayer on a GaN layer. The device also includes a first contact and asecond contact. The average thickness of the AlGaN layer varies betweenthe first contact and the second contact, for modulating the density ofan electron gas in the GaN layer between the first contact and thesecond contact.

According to another aspect of the invention, there is provided a methodof making a semiconductor device. The method includes forming an GaNlayer on a substrate. The method also includes forming an AlGaN layer onthe GaN layer. The method further includes forming a first contact and asecond contact of the device. The method also includes varying theaverage thickness of the AlGaN layer between the first contact and thesecond contact, for modulating the density of an electron gas in the GaNlayer between the first contact and the second contact.

In a semiconductor device having an AlGaN layer on a GaN layer, thedensity of the electron gas in the GaN layer is sensitive to thethickness of the AlGaN layer (Appl. Phys. Lett., Vol. 77, No. 2, 10 Jul.2000). The thicker the AlGaN barrier layer (for a given Al content), thestronger will be the voltage drop given by the existence of polarizationcharges at the top and bottom interfaces of this barrier layer, which inreturn will increase the depth of the quantum well formed in the GaNnear the AlGaN/GaN interface. This leads to a higher sheet carrierdensity of the electron gas and a lower sheet resistance.

In accordance with this invention, it has been realised that this effectcan be used to modulate the density of the electron gas between thecontacts of a semiconductor device. The modulation of the electron gasin turn produces a device having a resistivity that varies between thosecontacts. As described herein, the modulation profile of the electrongas between the contacts can take a number of different forms.

The variation in the average thickness of the AlGaN layer can beimplemented in a number of ways. For example, at least part of an uppersurface of the AlGaN layer can include a plurality of recesses forvarying an average thickness of the AlGaN layer. These recesses can beformed using standard semiconductor processing techniques such alithography and surface etching. It is noted that the recesses can beformed using a single lithographic/etch sequence, minimising the impactof the recesses on existing process flows. The well established natureof these techniques also allows great flexibility in tailoring theconfiguration of the recesses. For example the density (e.g. pitch),width or depth of the recesses can be varied between different areas ofthe AlGaN layer, whereby the average thickness of the layer is alsovaried. In one embodiment, the depth of the recesses is between 30% and100% of the local thickness of the AlGaN layer.

As used herein, the term “average thickness” refers not to the localthickness of the AlGaN layer at any given position, but instead to themacroscopic thickness of the AlGaN layer, averaged over a given area.For example, it will be appreciated that the local thickness of theAlGaN layer can vary dramatically in areas where the layer is providedwith the above mentioned recesses. Nevertheless, the average thicknessof the AlGaN layer in those areas will take a certain value which variesonly in the macroscopic sense, as parameters such as the density, sizeor depth of the recesses varies across the layer. The term “averagethickness” accordingly refers to the thickness of the AlGaN layer in thelatter, macroscopic sense.

The density, width or pitch of the recesses can vary across the surfaceof the AlGaN layer. The depth of the recesses can also be varied. Theaverage thickness T of the AlGaN layer between the first contact and thesecond contact can be in the range 10 nm<T<40 nm.

In addition to the flexibility in the density and/or depth of therecesses, there is also flexibility in the shape of the recesses. Forexample, the recesses could be provided in the form of dimples orgrooves. As noted above, the density of the dimples or grooves (i.e. thespacing between adjacent recesses) can be varied to vary the macroscopicaverage thickness. Recesses in the form of grooves can be provided inthe form of a grid.

In some embodiments, the semiconductor device can comprise a HighElectron Mobility Transistor (HEMT) having a source, a gate and a drain.In these embodiments, the first contact can comprise the source, and thesecond contact can comprise the drain.

In one embodiment, the AlGaN layer can have a larger average thicknessbetween the source and the gate than between the gate and the drain.This results in a device in which the sheet resistance of the electrongas between the source and the gate is lower than the sheet resistanceof the electron gas between the gate and the drain (owing to thedifferences in the electron gas densities in those regions caused by thechange in thickness of the AlGaN layer). The modulation of the electrongas density (and consequently the sheet resistance of the electron gas)in this way thus allows an improved trade-off between the on-stateresistance and breakdown voltage of the device.

In some examples, the thickness of the AlGaN layer can be substantiallyconstant between the source and the gate. In such examples, steps takento varying the average thickness between the source and the drain can berestricted to the gate-drain side of the device.

In accordance with an embodiment of the invention, the average thicknessof the AlGaN layer can increase from the gate to the drain. The increasein thickness of the AlGaN layer increases the density of the electrongas towards the drain, while suppressing it towards the gate. Theresulting gradual profile of sheet resistance of the electron gas fromthe gate (on the gate-drain side of the device) to the drain enhancesthe performance of the device and allows a better trade-off between theon-state resistance and breakdown voltage.

The variation in average thickness of the AlGaN layer between the gateand the drain can take a number of different forms. For example, thethickness can increase monotonically from the gate to the drain (that isto say, the average thickness rises between the gate and the drainwithout falling at any point). The thickness increase can occur in anumber of steps, or alternatively there can be a smooth transition (e.g.linear).

In one embodiment, the semiconductor device can be a Schottky barrierdiode. In this embodiment, the first contact can comprise the anode ofthe Schottky barrier diode, and the second contact can comprise thecathode. In a manner analogous to that described above in respect of theHEMT device, the average thickness of the AlGaN layer in the Schottkybarrier diode can increase from the anode to the cathode. The increasecan be either linear or take the form of a series of one or more steps.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described hereinafter, byway of example only, with reference to the accompanying drawings inwhich like reference signs relate to like elements and in which:

FIG. 1 shows a semiconductor device including an AlGaN layer on a GaNlayer;

FIG. 2 shows a semiconductor device comprising a High Electron MobilityTransistor (HEMT) according to an embodiment of the invention;

FIG. 3 shows a semiconductor device comprising a High Electron MobilityTransistor (HEMT) according to another embodiment of the invention;

FIG. 4 shows a semiconductor device comprising a High Electron MobilityTransistor (HEMT) according to a further embodiment of the invention;

FIG. 5 shows a semiconductor device comprising a High Electron MobilityTransistor (HEMT) according to another embodiment of the invention; and

FIGS. 6 and 7 show a semiconductor device comprising a Schottky barrierdiode according to a further embodiment of the invention.

DETAILED DESCRIPTION

Embodiments of the present invention are described in the following withreference to the accompanying drawings.

FIG. 1 illustrates an example of a High Electron Mobility Transistorsemiconductor device 10 including an AlGaN layer 6 on a GaN layer 4.These layers are typically grown epitaxially on a semiconductorsubstrate 2, which may, for example, comprise SiC. On an upper surfaceof the AlGaN layer there are provided a source 12, a gate 16 and a drain14. The source 12 and drain 14 include ohmic contacts, while in thisexample the gate 16 comprises a Schottky contact.

As described previously, the lattice mismatch between the AlGaN layer 6and the GaN layer 4 leads to the formation of an electron gas 8 in theGaN layer 4, near to the interface between the GaN layer 4 and the AlGaNlayer 6. The mobility of the electron gas 8 in the GaN layer 4 isrelatively high, which allows devices of the kind illustrated in FIG. 1to have very low resistance between the source 12 and the drain 14. Asis well known in the art, the channel resistance, determined by theelectron gas below the gate 16 can, in use, be altered by theapplication of a potential to the gate 16.

FIG. 2 illustrates a HEMT semiconductor device 100 in accordance with afirst embodiment of the invention. The device 100 includes a number ofthe features described above in relation to the device 10 of FIG. 1.Thus, the device 100 includes a semiconductor substrate 2 (e.g.comprising SiC, Al2O3, Si or any other substrate on which GaN can beepitaxially grown or bonded) on which a GaN layer 4 is provided. AnAlGaN layer 6 is provided on the GaN layer 4. In some examples, a bufferlayer may be located in between the GaN layer 4 and the AlGaN layer 6.Typically, the GaN layer 4, the AlGaN layer 6 and further layers such asa buffer layer can be grown on the major surface of the substrate 2using epitaxial growth techniques.

Also in common with the example shown in FIG. 1, the device 100 includesa source 12, a gate 16 and a drain 14. In the present example, thesource 12 and drain 14 include ohmic contacts, while the gate 16includes a Schottky contact. Optionally, the gate may be provided with adielectric layer (i.e. a non-Schottky gate). The features of the source12, gate 16 and drain 14 can be conventional in their nature, and willbe well recognised by the person skilled in the art.

The AlGaN layer 6 of the device 100 has an average thickness whichvaries between the source 12 and the drain 16. The average thickness ofthe AlGaN layer 6 in the devices described herein may vary, for example,from 10 nm to 40 nm.

In general, variations in the average thickness of the AlGaN layer 6 canbe used to modulate the density of the electron gas 8 which is locatedin the GaN layer 4, near to the interface between the GaN layer 4 andthe AlGaN layer 6. In the present example, the average thickness of theAlGaN layer 6 is greater on the source-gate side 20 of the device 100than on the gate-drain side 22 of the device 100. The modulation of theelectron gas density (and consequently the sheet resistance of theelectron gas) in this way allows an improved trade-off between theon-state resistance and the breakdown voltage of the device. Asdescribed below in relation to FIGS. 3 to 5, this trade-off can befurther improved in particular by reducing the average thickness of theAlGaN layer close to the gate 16 on the gate-drain side 22 of the device100.

In the present embodiment (FIG. 2), the average thickness of the AlGaNlayer 6 is substantially constant on the source-gate side 20 of thedevice 100. Consequently, the density of the electron gas 8 in the GaNlayer 4 is substantially uniform on the source-gate side 20 of thedevice 100.

As shown in FIG. 2, AlGaN layer 6 of the device 100 has an averagethickness on the gate-drain side 22 that is smaller than on thesource-gate side 20. This change in thickness could, in principal, beachieved by masking the device 100 on the source-gate side 20 andetching back the AlGaN layer 6 on the gate-drain side 22. In such anexample, the thickness of the AlGaN layer 6 on the gate-drain side 22would, like the thickness of the AlGaN layer 6 on the source-gate side20, be substantially constant, albeit thinner.

However, in accordance with an embodiment of this invention, it has beendetermined that the average thickness of the AlGaN layer 6 can also bevaried by providing the AlGaN layer 6 with a series of recesses 30 on anupper surface thereof. As mentioned above, although the thickness of theAlGaN layer 6 in such examples varies locally between the recessed andnon-recessed portions thereof, the overall effect is that the average(macroscopic) thickness of the AlGaN layer 6 is lessened by the presenceof the recesses 30. This still achieves the effect of reducing theelectron gas density in the GaN layer 4 by allowing the lattice of theAlGaN layer 6 to relax to a certain degree, weakening the mismatcheffect described above. Additionally however, use of recesses such asthose described herein provides a high degree of flexibility for varyingthe thickness of the AlGaN layer 6 across the device 100. This isbecause the configuration (e.g. depth, width, shape, pitch) of therecesses 30 can be defined in a highly controlled manner usinglithography and etching steps.

Returning to the Example of FIG. 2, the effect of the recesses 30,reducing the average thickness of the AlGaN layer 6 on the gate-drainside 22, lowers the electron density in the electron gas 8 in the region80 beneath the recesses. It is noted that the recesses in the Figures ofthis application are shown only schematically. In accordance with anembodiment of the invention, in order to guarantee a macroscopicallydefined, smoothed out sheet resistance, both the spacing and/or width ofthe grooves should be comparable to the thickness of the AlGaN barrierlayer. For example, the spacing and/or width of the grooves can be fromone to four times the thickness of the AlGaN barrier layer.

FIGS. 3 to 5 illustrate HEMT devices 100 in accordance with furtherembodiments of the invention, viewed from above the substrate 2. It isenvisaged that the cross sectional arrangement of these furtherembodiments may be broadly similar to that shown in FIG. 2, except forthe configuration of the recesses 30 and the corresponding densities inthe electron gas 8 in the GaN layer 4 as described in more detail below.

The device 100 shown in FIG. 3 includes recesses 30 that are provided inthe form of grooves in the upper surface of the AlGaN layer. Inalternative embodiments, the recesses may take different forms. Forexample, the recess could be provided in the form of a plurality ofdimples in the upper surface of the AlGaN layer 6. The dimples may bearranged in a regular pattern or array, such that the average thicknessof the AlGaN layer 6 does not vary unintentionally from area to area. Interms of local thickness, the depth of the recesses can be between 30%and 100% of the local thickness of the AlGaN layer 6.

As noted above, the grooves shown in FIG. 3 can be formed usinglithography and etching techniques. In the present example, the groovesare provided in a grid like configuration. The grid itself is positionedadjacent the gate 16, and need not extend all the way to the drain 14 toachieve the improvement in the trade-off between the on-state resistanceand the breakdown voltage of the device noted above. As shown in FIG. 3,an area 34 of the surface of the AlGaN layer 6 on the gate-drain side 22of the device 100 may be free from recesses. In the present example,that area 34 is adjacent the drain 14.

The spacing of the recesses 30 can be chosen in accordance with thedesired average thickness of the AlGaN layer 6. Additionally, inprincipal it is also possible to vary the depth of the recesses to tunethe average thickness of the AlGaN layer 6. However, manufacture of adevice having recesses of various depths would be relatively complicatedin comparison simply varying the recess spacing or width, since multiplemasks and etching steps would be required.

FIG. 4 illustrates an example of a HEMT device 100 in which recesses areprovided which have different spacings therebetween, in different areasof the surface of the AlGaN layer 6 (i.e. the density of the recesses onthe surface of the AlGaN layer 6 varies). In particular, the density ofthe recesses 30 (comprising grooves in a grid configuration in thepresent example), is higher in an area closest to the gate 16. In aneighbouring area 32, the density of the recesses is somewhat lower.Finally, in an area 34 adjacent the drain 14, the recesses are absent.Thus, the average thickness of the AlGaN layer 6 on the gate-drain side22 of the device varies in a step-like manner from a thinnest areaadjacent the gate, via an area 32 having intermediate thickness, to athickest area 34 which is adjacent the drain 16. This variation in theaverage thickness of the AlGaN layer 6 allows the electron density inthe gas 8 to be varied accordingly. Once again, these variations can beused to tune the trade-off between the on-state resistance and thebreakdown voltage of the device.

FIG. 5 illustrates a further example of a HEMT device 100 in accordancewith an embodiment of the invention. In contrast with the device 100shown in FIG. 4, in which the density of the recesses 30 on the surfaceof the AlGaN layer 6 vary in a step like manner, in the present examplethe density of the recesses varies substantially continuously in aregion 36 on the gate-drain side 22. The average thickness of the AlGaNlayer 6 varies in a correspondingly continuous manner, as does the sheetdensity and sheet resistance of the electron gas 8 in the GaN layer.This continuous variation in density can be achieved in the manner shownin FIG. 5, using changes in the spacings or widths between neighbouringgrooves arranged in a grid. Alternative embodiments are envisagedhowever, such as a continuous change in the spacing between neighbouringdimples provided in the AlGaN layer 6 surface. It is also envisaged thata combination of the stepped approach and the continuously varyingapproach described above could be employed in different regions of thesame device.

FIGS. 6 and 7 illustrate a semiconductor device 200 comprising aSchottky barrier diode, in accordance with an embodiment of theinvention. The device 200 includes a first contact 66 forming an anodeof the diode, and a second contact 64 forming a cathode. As can be seenfrom FIG. 7, the layer structure of the device 200 is similar to that ofthe HEMT devices described above. Thus, the device 200 includes asubstrate 52 (which, as noted above, can comprise SiC, Al2O3, Si or anyother substrate on which GaN can be epitaxially grown or bonded), a GaNlayer 54 and an AlGaN barrier layer 56.

As with the HEMT devices described above, an electron gas 8 forms in theGaN layer 54 near the interface between the GaN layer and the AlGaNlayer 56. The carrier density of the gas 8 can be modulated between theanode and the cathode in much the same way as described for the HEMTdevices.

Accordingly, as shown in FIGS. 6 and 7, a plurality of recesses 50 canbe provided on an upper surface of the AlGaN layer 56. The recesses canbe configured in accordance with any of the above described HEMT relatedexamples in terms of shape, depth, width, density (pitch) and location.In the example of FIGS. 6 and 7, the recesses are provided in a gridarrangement adjacent the anode 66, while an area 51 of the surface ofthe AlGaN layer 56 adjacent the cathode 64 is free from recesses. Asshown in FIG. 7, this arrangement of recesses lowers the electrondensity in the electron gas 8 in the region 90 beneath the recesses,while a higher density remains beneath the region 51 adjacent thecathode 64. Thus the carrier density increases from the anode 66 to thecathode 64.

Accordingly, there has been described a semiconductor device and amethod of making the same. The device includes a semiconductor substratehaving an AlGaN layer on a GaN layer. The device also includes firstcontact and a second contact. The average thickness of the AlGaN layervaries between the first contact and the second contact, for modulatingthe density of an electron gas in the GaN layer between the firstcontact and the second contact.

Although particular embodiments of the invention have been described, itwill be appreciated that many modifications/additions and/orsubstitutions may be made within the scope of the claimed invention.

1. A semiconductor device comprising: a semiconductor substrateincluding an AlGaN layer on a GaN layer; a first contact, and a secondcontact, wherein the average thickness of the AlGaN layer varies betweenthe first contact and the second contact, for modulating the density ofan electron gas in the GaN layer between the first contact and thesecond contact.
 2. The device of claim 1, wherein at least part of anupper surface of the AlGaN layer includes a plurality of recesses forvarying an average thickness of the AlGaN layer.
 3. The device of claim2, wherein a density of the recesses varies for varying the averagethickness of the AlGaN layer.
 4. The device of claim 2, wherein thedepth of the recesses is between 30% and 100% of the local thickness ofthe AlGaN layer.
 5. The device of claim 2, wherein the recesses arearranged in a regular array.
 6. The device of claim 2, wherein therecesses comprise grooves.
 7. The device of claim 1, wherein the averagethickness T of the AlGaN layer between the first contact and the secondcontact is in the range 10 nm<T<40 nm.
 8. The device of claim 1comprising a High Electron Mobility Transistor (HEMT) having a source, agate and a drain, wherein the first contact comprises said source, andwherein the second contact comprises said drain.
 9. The device of claim8, wherein the AlGaN layer has a larger average thickness between thesource and the gate than between the gate and the drain.
 10. The deviceof claim 8, wherein the average thickness of the AlGaN layer increasesfrom the gate to the drain.
 11. The device of claim 10, wherein theaverage thickness of the AlGaN layer increases either linearly or in aseries of one or more steps from the gate to the drain.
 12. The deviceof claim 1, wherein the first contact comprises the anode of a Schottkybarrier diode, and wherein the second contact comprises the cathode ofsaid Schottky barrier diode.
 13. The device of claim 12, wherein theaverage thickness of the AlGaN layer increases from the anode to thecathode.
 14. The device of claim 13, wherein the average thickness ofthe AlGaN layer increases either linearly or in a series of one or moresteps from the anode to the cathode.
 15. A method of making asemiconductor device, the method comprising: forming an GaN layer on asubstrate; forming an AlGaN layer on the GaN layer; forming a firstcontact and a second contact of the device; and varying the averagethickness of the AlGaN layer between the first contact and the secondcontact, for modulating the density of an electron gas in the GaN layerbetween the first contact and the second contact.